Method for production of high-strength low-expansion cast iron

ABSTRACT

A method is proposed for the production of high-strength low-expansion cast iron enabled to acquire improved strength, hardness, and cutting workability while retaining the property of low expansion intact. The product is a low-expansion cast iron having a high nickel content and exhibiting a coefficient of thermal expansion of not more than 8×10 -6  /° C. at temperatures in the range of from room temperature to 100° C. By causing a carbide to be finely precipitated in an area ratio in the range of from 0.3% to 20% in the metal structure of the cast iron and lowering the C content in the cast iron, there is produced a high-strength low-expansion cast iron. The deposition of the carbide mentioned above is accomplished by incorporating in the material for cast iron at least one element selected from the group consisting of the transition metal elements of IVa, Va, and VIa Groups in the Periodic Table of the Elements.

TECHNICAL FIELD

This invention concerns a low-expansion cast iron having a high Nicontent and relates to a method for the production of a high-strengthlow-expansion cast iron which is allowed to acquire exalted strengthwithout a sacrifice of the low-expansion property inherent therein.

BACKGROUND ART

As known to date, cast iron has been in popular use as the basicmaterial for industry. This is because the cast iron has such advantagesas excelling in castability, allowing formation of multiple kinds ofcomplicatedly shaped articles, readily yielding to cutting and similarmachining works, incurring rather low expenses in procurement of rawmaterials and execution of melting work, and enjoying ease ofmanufacture even at a factory of a small scale.

Recently, the electronic industry and the optical industry have advancedto a point where the machine tool, measuring devices, molding dies, andother manufacturing machines which are associated with these industriesdemand materials of increasingly high accuracy and function. For thepurpose of answering this demand, the necessity for materials which arecapable of lowering thermal expansion coefficient and repressing thermaldeformation to the fullest possible extent besides keeping thecharacteristic properties of the conventional materials intact isgrowing all the more in profundity. As metallic materials of low thermalexpansion coefficients, an about 36%Ni--Fe alloy (Invar alloy) and anabout 30%Ni--5%Co--Fe alloy (Super Invar alloy) which are shown in Table1-1 and Table 1-2 are known. They have not yet been fully tamed for theutmost use. This is because they are unfortunately deficient in cuttingworkability, and castability. In recent years, the materials which areobtained by treating the Invar and the Super Invar alloy so as to impartthe quality of cast iron thereto and vest them with improved cuttingworkability and enhanced castability and which, therefore, are relievedof the drawback mentioned above have been attracting growing attention.Table 1-1 and Table 1-2 also show the low-expansion cast iron which hasbeen known as Niresist D5 for a long time, Nobinite cast iron as oneexample of the low-expansion cast irons developed in the last severalyears, and the cast iron disclosed in JP-A-62-268,249.

The materials shown in Table 1-1 and Table 1-2, however, are eitheralloys which have not induced separation of graphite by crystallizationor nodular graphite cast irons and mainly have an austenitic structureas a base matrix and, therefore, have tensile strength in the range offrom 40 to 55 kgf/mm² and Brinell hardness in the neighborhood of HB120. Where the graphitic structure is formed of graphite flakes orpseudonodular graphite particles, the tensile strength is still lower inthe approximate range of from 25 to 35 kgf/mm² and the Brinell hardnessin the neighborhood of HB 100. When they are applied to such parts asare required to have high accuracy, therefore, the produced parts oftenpose problems of deformations of various sorts due to insufficientstrength. Owing to the softness, they find only limited applications tosuch sliding parts as are in need of resistance to abrasion.

Besides the materials cited above, JP-A-61-177,356 discloses a lowthermal expansion high-nickel content austenite graphite cast iron ofthe shape of vermicular, JP-A-02-298,236 an alloy having low thermalexpansion at a relatively high temperature, JP-A-64-55,364 a low thermalexpansion cast iron endowed with improved strength by a heat treatment,JP-B-01-36,548 a low thermal expansion alloy incorporating Ni, Co, V,and Nb therein, JP-A-02-70,040 a low thermal expansion alloy endowedwith improved strength by a solid solution treatment, and JP-A-63-433 agraphite cast iron of the shape of vermicular. None of them satisfiesboth high strength and low expansion; some of them are deficient instrength and others in low expansion.

It is further known that low expansion cast irons having a graphitestructure generally incur conspicuous Ni segregation and, because of theliability to have a low Ni concentration in the gap of the Dendritephase, produce a part deviating from the Invar composition and sufferfrom deficiency in low expansion as compared with the Invar alloy andthe Super Invar alloy which form no graphite. Generally, this problem ofNi segregation is solved by the method of subjecting the low expansioncast iron to a solid solution treatment at a temperature in the range offrom 750° C. to 950° C. and then to rapid cooling. This method, however,entails the problem of causing the heat-treated cast iron to deform.Particularly in the case of a low thermal expansion cast iron, since itis an alloy of high Ni content, it has low thermal conductivity ascompared with ordinary cast iron and, when hardened in water or oil,shows a large difference in cooling speed between the surface layer andthe inner part of a shaped part of the low expansion cast iron andconsequently gives rise to a large stress of heat treatment. Thus, theshaped part is destined to retain residual stress if not suffered toinduce growth of deformation. Further, since this residual stress isliberated during the course of mechanical fabrication or with the elapseof time, the shaped part of the low expansion cast iron brings aboutdegradation of shape or dimensional accuracy. As a result, it has beennecessary for the heat-treated cast iron to undergo a protracted heattreatment which is adapted for the relief of strain.

In association with the recent trend of the products of cast iron towardgrowth in size and complication in shape, the present inventors havetaken notice of the fact that the heat treatment which is given afterthe step of casting inevitably impairs the reliability of the products.It has been ascertained to them, for example, that the heat treatmentwhich has brought about a favorable effect on the conventional surfaceplate having 55 cm in diameter and 40 mm in thickness brings about anunfavorable effect of impairing the flatness of surface of a surfaceplate having 1 m in diameter and 40 mm in thickness.

In the case of such products as are complicated in shape, since theyhave been fabricated heretofore by machining, the strain which isgenerated by stress within these products in the process of fabricationhas likewise posed a problem. To be relieved of this strain, theseproducts must undergo a time-consuming strain-relieving heat treatment.By reason of the complicatedness of this heat treatment, thedesirability of cast products manufacturable without requiring the heattreatment has been finding popular approval. The improvement which isattained in the low expansion property by the heat treatment(particularly for rapid cooling) possibly exerts an adverse effect onthe improvement of the dimensional accuracy which constitutes theprimary object of the heat treatment. Thus, in the case of the productsof complicated shapes which have been heretofore manufactured bymachining because the strain-relieving heat treatment applicable theretois unduly intricate, the desirability of obtaining these products bycasting without entailing development of strain due to stress has beenfinding approval. The cast products, therefore, are desired to retaintheir inherently low expansibility as cast as much as possible.

DISCLOSURE OF THE INVENTION

In the existing circumstance that machines of various kinds are tendingtoward increasingly large dimensions, increasingly complicated shapes,and increasingly high operational accuracy, the conventionallow-expansion cast iron in no infrequent cases fails to adapt fully tosuch machines in terms of mechanical strength, hardness, or the like.The semiconductors which have been produced in recent years, forexample, have markedly increased numbers of components per chip.Consequently, the Si wafers to be used for the semiconductors arerequired to possess surface flatness of increasingly high accuracy.Meanwhile, the Si wafers have been tending year after year towardincreasing diameters. They are said to be verging on the stage oftransition from 4- to 5-inch discs to 8-inch discs. Under thiscircumstance, polishing surface plates made of low-expansion cast ironhave been finding growing adoption for the purpose of machining the Siwafers. Since the production of Si wafers in an increased diameternaturally urges these polishing surface plates toward growth in size,the cast iron for use in the polishing surface plates is required topossess tensile strength of not less than 55 kgf/cm² indispensable tothe retention of the accuracy of shape besides satisfying lowexpansibility.

In consideration of the possible use of this low-expansion cast iron insliding parts, for example, the cast iron is desired to possess enhancedhardness for the purpose of enabling the sliding parts to manifestexalted resistance to abrasion. Since the hardness also affects theproperty of cuttability, the cast iron is desired to acquire a suitabledegree of hardness for the sake of improving the cuttability.

Specifically, the cast iron to be obtained by the method of productionaccording to this invention is required to possess the followingproperties.

Firstly, the cast iron requires to show low expansibility. According tothe results of the inventors' study and with due consideration for thesecond through the fourth property, it is concluded that the cast ironis desired to have a coefficient of thermal expansion of not more than8×10⁻⁶ at temperatures in the range of from room temperature to 100° C.

Secondly, the cast iron requires tensile strength. In the light of theresults of the inventors' study, it is concluded that the cast iron isdesired to have tensile strength of not less than 55 kgf/mm² to keep theshape and size thereof intact in addition to satisfying the coefficientof thermal expansion mentioned above.

Thirdly, the cast iron requires abrasion resistance, namely hardness. Itis desired to have Brinell hardness of not less than 200 to acquiredesired abrasion resistance in addition to satisfying the thermalexpansion and the tensile strength mentioned above.

Fourthly, the cast iron requires such cutting workability andcastability as are proper for any cast iron.

Now, the advantages of the fact that a cast product is a material ascast will be described below.

Generally, a cast product is vested with a desired property by a heattreatment which is performed after the step of casting. This heattreatment inflicts residual stress on the interior of the cast product.Normally, this cast product is subjected to a strain relieving heattreatment to be relieved of this residual stress. This heat treatment,however, proves complicated from the operational point of view and, attimes, fails to attain the removal of residual stress, depending on theparticular kind of product, as remarked above. To avoid this problem,therefore, the cast iron is desired to be a material as cast.

During this heat treatment, the cast product must not suffer degradationof the four properties mentioned above. In addition to satisfyingsimultaneously the four properties mentioned above, the cast product isdesired to be a material as cast.

This invention, produced for the purpose of coping with the variousproblems remarked above, has for an object thereof the provision of amethod for the production of high-strength low-expansion cast iron whichis endowed with enhanced strength and hardness and also with improvedcutting workability and meanwhile enabled to keep low expansibilityintact.

The present invention has for another object thereof the provision of amethod for the production of high-strength low-expansion cast ironinfallibly endowed with low expansibility without undergoing such a heattreatment as the quench hardening which is effected by a sudden fall oftemperature from a high to a low level.

This invention provides a method for the production of high-strengthlow-expansion cast iron, more particularly a method for the productionof low-expansion cast iron of a high Ni content exhibiting a coefficientof thermal expansion of not more than 8×10⁻⁶ /° C. at temperatures inthe range of from room temperature to 100° C., characterized by thesteps of preparing a material consisting of not less than 0.3% by weightto not more than 2.5% by weight of C, not more than 0.1% by weight of Mgor Ca, not less than 25% by weight to not more than 40% by weight of Ni,less than 12% by weight of Co, not less than 0.1% by weight to not morethan 6.0% by weight of a carbide-forming element, and the balance of Feand other inevitable impurities, melting the material and casting themelt in a mold of a stated shape, and enabling the carbide-formingelement, while the melt is being solidified in the mold, to beprecipitated in the form of a carbide at an area ratio in the range offrom 0.3% to 20% in the metal structure.

The material of the aforementioned composition for the cast iron furtherincorporates therein not more than 1.2% by weight of Si for the sake ofimparting castability and cuttability and not more than 1.0% by weightof Mn for the sake of promoting deoxidation, enhancing strength, andimproving resistance to corrosion.

The carbide-forming element mentioned above is at least one element tobe selected from the group consisting of the transition metallicelements of Groups IVa, Va, and VIa in the Periodic Table of theElements.

This invention further provides a method for the production ofhigh-strength low-expansion cast iron, more particularly a method forthe production of low-expansion cast iron of a high Ni contentexhibiting a coefficient of thermal expansion of not more than 8×10⁻⁶ /°C. at temperatures in the range of from room temperature to 100° C.,characterized by the steps of preparing a material consisting of notless than 0.3% by weight to not more than 2.5% by weight of C, not morethan 0.1% by weight of Mg or Ca, not less than 25% by weight to not morethan 40% by weight of Ni, less than 12% by weight of Co, not less than0.1% by weight to not more than 6.0% by weight of a carbide-formingelement, and the balance of Fe and other inevitable impurities, meltingthe material and casting the melt in a mold of a stated shape, andenabling the carbide-forming element, while the melt is being solidifiedin the mold, to be precipitated in the form of a carbide therebylowering the content of dissolved carbon in the cast iron to not morethan 0.4% by weight.

The material of the composition for the cast iron mentioned abovefurther incorporates therein not more than 1.2% by weight of Si for thesake of imparting castability and cuttability and not more than 1.0% byweight of Mn for the sake of promoting deoxidation, enhancing strength,and improving resistance to corrosion.

The carbide-forming element mentioned above is at least one element tobe selected from the group consisting of the transition metallicelements of Groups IVa, Va, and VIa in the Periodic Table of theElements.

This invention further provides a method for the production ofhigh-strength low-expansion cast iron, characterized by the steps ofpreparing a material consisting of not less than 0.3% by weight to notmore than 2.5% by weight of C, not more than 0.1% by weight of Mg or Ca,not less than 25% by weight to not more than 40% by weight of Ni, lessthan 12% by weight of Co, not less than 0.1% by weight to not more than6.0% by weight of a carbide-forming element, and the balance of Fe andother inevitable impurities, melting the material and casting the meltin a mold of a stated shape, and enabling the carbide-forming element,while the melt is being solidified in the mold, to be precipitated inthe form of a carbide in the metal structure thereby lowering thecontent of dissolved carbon in the cast iron and producing low-expansioncast iron exhibiting a coefficient of thermal expansion of not more than8×10⁻⁶ /° C. at temperatures in the range of from room temperature to100° C. and tensile strength of not less than 55 kgf/mm².

The material of the composition for the cast iron mentioned abovefurther incorporates therein not more than 1.2% by weight of Si for thesake of imparting castability and cutting workability and not more than1.0% by weight of Mn for the sake of promoting deoxidation, enhancingstrength, and improving resistance to corrosion.

The carbide-forming element mentioned above is at least one element tobe selected from the group consisting of the transition metallicelements of Groups IVa, Va, and VIa in the Periodic Table of theElements.

The carbide-forming element is precipitated in the form of a carbide atan area ratio in the range of from 0.3% to 20% in the metal structure.

The content of the dissolved carbon in the cast iron is not more than0.4% by weight.

A method for the production of a polishing surface plate, ischaracterized by the steps of preparing a material consisting of notless than 0.3% by weight to not more than 2.5% by weight of C, not morethan 0.1% by weight of Mg or Ca, not less than 25% by weight to not morethan 40% by weight of Ni, less than 12% by weight of Co, not less than0.1% by weight to not more than 6.0% by weight of a carbide-formingelement, and the balance of Fe and other inevitable impurities, meltingthe material and casting the melt in a mold of a stated shape, andenabling the carbide-forming element, while the melt is being solidifiedin the mold, to be precipitated in the form of a carbide at an arearatio in the range of from 0.3% to 20% in the metal structure.

The material of the composition for the cast iron of the polishingmachine further incorporates therein not more than 1.2% by weight of Sifor the sake of imparting castability and cuttability and not more than1.0% by weight of Mn for the sake of promoting deoxidation, enhancingstrength, and improving resistance to corrosion.

The carbide-forming element mentioned above is at least one element tobe selected from the group consisting of the transition metallicelements of Groups IVa, Va, and VIa in the Periodic Table of theElements.

The polishing surface plate has a large bulk not less than 600 mm indiameter and, in one aspect, is characterized by the fact that it isobtained as cast and obviates the necessity of undergoing a heattreatment after the casting.

The cast iron mentioned above is characterized in that the content ofthe dissolved carbon in the cast iron is lowered to not more than 0.4%by weight.

The method of production mentioned above is characterized by allowingproduction of a high-strength low-expansion cast iron polishing machinewhich is formed of cast iron having a coefficient of thermal expansionof not more than 8×10⁻⁶ /° C. at temperatures in the range of from roomtemperature to 100° C. and tensile strength of not less than 55 kgf/mm².

A method for the production of a rod for use in a laser oscillator, ischaracterized by the steps of preparing a material consisting of notless than 0.3% by weight to not more than 2.5% by weight of C, not morethan 0.1% by weight of Mg or Ca, not less than 25% by weight to not morethan 40% by weight of Ni, less than 12% by weight of Co, not less than0.1% by weight to not more than 6.0% by weight of a carbide-formingelement, and the balance of Fe and other inevitable impurities, meltingthe material and casting the melt in a mold of a stated shape, andenabling the carbide-forming element, while the melt is being solidifiedin the mold, to be precipitated in the form of a carbide at an arearatio in the range of from 0.3% to 20% in the metal structure.

In the method for the production of a rod for use in a laser oscillatoraccording to this invention, the material of the composition for thecast iron of the rod further incorporates therein not more than 1.2% byweight of Si for the sake of imparting castability and cuttability andnot more than 1.0% by weight of Mn for the sake of promotingdeoxidation, enhancing strength, and improving resistance to corrosion.

The carbide-forming element mentioned above is at least one elementselected from the group of the transition metallic elements of GroupsIVa, Va, and VIa in the Periodic Table of the elements.

Further, the method for the production of a rod for use in a laseroscillator according to this invention is characterized by not includinga heat treatment subsequent to the step of casting.

The cast iron mentioned above is characterized in that the content ofthe dissolved carbon in the cast iron is lowered to not more than 0.4%by weight.

The method of production mentioned above is characterized by allowingproduction of a high-strength low-expansion cast iron rod for use in alaser oscillator which is formed of cast iron having a coefficient ofthermal expansion of not more than 8×10⁻⁶ /° C. at temperatures in therange of from room temperature to 100° C. and tensile strength of notless than 55 kgf/mm².

The invention described above has been perfected on the basis of thefollowing knowledges. The avoidance of impairment of the property of lowexpansion to the fullest possible extent has been the first condition ofthis invention. In other words, the essence of the present inventionconsists in using the basic composition of a Super Invar alloy(30%Ni--5%Co--65%Fe) as a base metal and repressing the content of solidsolutions with other elements in the matrix to the fullest possibleextent. To be more specific, the present inventors have acquired aknowledge that the desired property of low expansion is obtained bylowering the content of dissolved carbon in the cast iron to not morethan 0.4% by weight. Since this is practical purpose cast iron having agraphitic structure, it naturally tolerates the presence of suchelements as C, Si, Mn, and Mg and impurities which are inevitablycontained therein. The coefficient of thermal expansion of thelow-expansion cast iron of the present invention is not more than 8×10⁻⁶/° C. at temperatures in the range of from room temperature to 100° C.

Then, for the sake of improving strength and hardness, this inventionhas the enhancement of the dispersion of a third phase for the secondcondition. This enhancement is attained by adding a carbide-formingelement as a dissolving component and inducing deposition of a carbidein the process of solidification. The present inventors have acquired anovel knowledge that owing to this mechanism, the dissolved carbon isconsumed in the form of a carbide and this consumption can be expectedto produce an effect of lowering the thermal expansion coefficient ofthe alloy. If the amount of the carbide-forming element is added in anexcess of the amount so consumed as the carbide, however, the excessadds itself to the solid solution and rather increases the thermalexpansion coefficient than decreases it. Thus, the amount of thisaddition must be proper.

The present inventors have further found the conditions under which ameans to retain intact the property of low expansion possessed by theiron alloy as cast without requiring any heat treatment for rapidlycooling the alloy from an elevated temperature is realized within thescope of the method mentioned above. They have been ascertained thatwhen the carbide and graphite are both formed during the solidificationin the process of casting, the amount of dissolved carbon in thesolidifying phase is generously lowered and the segregation of Ni isrepressed. In accordance with these conditions of formulation, there isobtained a method for the production of a high-strength low-expansioncast iron which, as cast, acquires the same property of low expansion asthe material which has undergone a rapidly cooling treatment and, avoidsthe change of size and shape with aging due to thermal deformation andthe relief of residual stress.

The knowledges described above have been confirmed by the followingexperimental data.

From copious experimental data shown in FIG. 7, the present inventorshave acquired a novel knowledge that the strength properties (tensilestrength, proof strength, Young's modulus, and hardness) of theconventional low-expansion cast iron which contains no carbide in themetal structure have a very close relation with the carbon content ofthe cast iron. FIG. 8 shows the relation between the total carboncontent and the dissolved carbon content. In the region in whichgraphite is crystallized in the cast material when the total carboncontent is not less than about 1%, the ratio of graphitization isheightened in proportion as the total carbon content is increased and,as a result, dissolved carbon content tends to decrease. In short, thestrength and hardness of a low-expansion cast iron are increased byincreasing the dissolved carbon content. However, since an increase inthe amount of dissolved carbon results in an increase in the thermalexpansion coefficient, it is difficult to satisfy both high strength andlow expansion at the same time.

This invention has issued from a novel knowledge that by effecting theformation of a carbide in the metal structure of a low-expansion castiron, the dissolved carbon content can be decreased to a far greaterextent than when no carbide is present as shown in FIG. 8.

In the method of this invention for the production of a high-strengthlow-expansion cast iron, nickel (Ni) is a component which contributes toaustenite the metal structure of cast iron and lower the thermalexpansion coefficient of the cast iron. The low-expansion cast iron isobtained effectively when the Ni content thereof is made to fall in therange of from 25 to 40% by weight. If the Ni content deviates in eitherway from this range, the thermal expansion coefficient will beincreased. The Ni content is preferably in the range of from 28 to 36%by weight.

Cobalt (Co) and Ni produce a synergistic effect of further lowering thethermal expansion coefficient of cast iron. If the cobalt contentexceeds 12% by weight, however, it will conversely increase the thermalexpansion coefficient. In the alloy as cast which has not undergone anyparticular heat treatment, Ni and Co are segregated therein andconsequently exert adverse effects on the property of low expansion. Cois to be added, therefore, in due consideration of the thermal expansioncoefficient and other factors which the cast iron is required topossess.

Carbon (C) is a component which induces crystallization of graphite inthe low-expansion cast iron and imparts castability, cuttability,workability, etc. to the cast iron. The carbon which has escapedgraphitization continues to exist as a carbide and the dissolved carboncontent. This invention features the improvement of the strength andhardness of a low-expansion cast iron by the formation of a carbide inthe metal structure thereof. In this respect, therefore, carbonconstitutes itself the most important component element. The excesscarbon is a carbon component for a dissolved carbon and forms a causefor an increase in the thermal expansion coefficient. It is, therefore,important to set the amount of carbon so as to lower the dissolvedcarbon content to the fullest possible extent. In this invention, thecarbon content is in the range of from 0.3 to 2.5% by weight. If thecarbon content is less than 0.3% by weight, no ample castability will beimparted. If the carbon content exceeds 2.5% by weight, the thermalexpansion coefficient will be unduly large. When the carbon content isin the range of from 0.3 to 1.0% by weight, no graphite is crystallizedbut a carbide is only formed in the cast iron which has not undergoneany heat treatment. In this case, the cuttability and property of lowexpansion can be improved by subjecting the cast iron to a heattreatment which is aimed at secondary graphitization. When the carboncontent is in the range of from 1.0 to 2.5% by weight, both graphite andthe carbide are formed in the cast iron as cast. Thus, the low-expansioncast iron consequently obtained excels in both cuttability and propertyof low expansion. Preferably, the carbon content is in the range of from1.0 to 1.5% by weight. By including the formation of a carbide for anadditional condition in this invention, therefore, the dissolved carboncontent in the solidifying phase can be kept at a low level and the Nisegregation can be repressed to a negligible extent. When the carboncontent is in this range, the cast iron as cast acquires a property oflow expansion close to that of a cast iron which has undergone a heattreatment with rapid cooling.

Silicon (Si) in this invention plays only meagerly the parts in thegraphitization of ordinary cast iron as offering sites for the formationof graphite cores and constituting a component equivalent to carbon. Inthe low-expansion cast iron of this invention, silicon is incorporatedfor the purpose of repressing the oxidation of cast iron during meltingin the open air. The silicon content, therefore, is desired to be as lowas possible. It is not more than 1.2% by weight, preferably not morethan 0.5% by weight.

Manganese (Mn) is one of the basic components of cast iron and functionsas a deoxidizing agent or an agent for enhancing strength and resistanceto corrosion. If it is contained in an unduly large amount, the excesswill increase the dissolved manganese content in the cast iron andproportionally enhance the thermal expansion coefficient. The Mncontent, therefore, is not more than 1.0% by weight, preferably not morethan 0.5% by weight.

Magnesium (Mg) or calcium (Ca) functions as a component for theformation of nodular graphite or as a deoxidizing agent for cast iron.Similarly to Mn, the upper limit of the Mg or Ca content is fixed at0.1% by weight for the purpose of preventing growth of thermal expansioncoefficient. Generally, Mg is used mainly. A Ni--5% Mg alloy or aFe--5%Mg alloy is added after the raw material blend has been melted andimmediately before the melt is cast and is consequently allowed to reactwith the melt. For the spheroidization of graphite, the cast iron aftersolidification generally requires to have a Mg or Ca content in therange of from 0.04 to 0.09%. If the Mg or Ca content is in the range offrom 0.01 to 0.03%, the graphite will assume the form of decayedspheroids called a psuedonodular graphite or CV cast iron graphite. Ifthe Mg and Ca contents have only effected deoxidization and remain inthe order of not more than 0.01%, the graphite will assume a flakegraphite. The property of low expansion is exalted and the strength isconversely degraded in proportion as the ratio of spheroidization ofgraphite decreases because the ratio of the amount of carbon transformedinto graphite in all the carbon content will be increased and the amountof dissolved carbon will be lowered.

As other impurities, phosphorus (P) and sulfur (S) are contained inpractical cast iron. Since they are undesirable contents for the purposeof this invention, their contents are desired to be as small aspossible. The total content of phosphorus and sulfur, therefore, is notmore than 0.2% by weight.

As the carbide-forming element, at least one element selected from thegroup of transient elements belonging to the IVa, Va, and VIa Groups inthe Periodic Table of the Elements, preferably one element selected fromamong Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W. This element is added in anamount in the range of from 0.1 to 6.0% by weight. These elements areinvariably the transition elements of the IVa, Va, and VIa Groups andhave low levels of free energy for the formation of a carbide in an ironalloy. The carbides of these elements are more liable to nucleate thangraphite. When the cast iron has a carbon content of not more than 0.9%,no graphite is formed and only a carbide is precipitated in the metalstructure. If the low-expansion cast iron contains no carbide-formingelement, there will arise such a carbon concentration gradient as hasthe lowest dissolved carbon content in the neighborhood of graphite anda high carbon content between graphites (dendrite gaps). As a result,the Ni which is expelled by the carbon is caused to form a concentrationgradient and give rise portions of a low Ni content between graphites(dendrite gaps) (reverse segregation). The present inventors, however,have found that the carbide-forming element mentioned above rathersegregates between graphites and forms a carbide and produces an effectof cancelling the concentration of gradient of the dissolved carboncontent due to the formation of graphite. They have further found thatthe precipitation of the carbide enhances the strength, Young's modulus,and hardness and decreases the dissolved carbon content and thecancellation of the Ni segregation exalts the property of low expansion.While the low-expansion cast iron relies on graphite to mend the defectof poor workability due to the stickiness of the austenite base matrixpeculiar to high-nickel cast iron, it has been found that theprecipitation of the carbide is effective in adjusting the stickinessand enhancing the workability.

The carbide-forming elements enumerated above can be used either singlyor in the form of a mixture of two or more members. The amount of thecarbide-forming element to be added is in the range of from 0.1 to 6.0%by weight at a total, though variable with the amount of carbon. If theamount of the carbide-forming element to be contained is less than 0.1%by weight, the carbide will not be sufficiently formed and the effectsmentioned above will not be obtained fully satisfactorily. Conversely,if the content of the carbide-forming element exceeds 6.0% by weight,the precipitated carbide will coarsen and not only fail to contribute tothe enhancement of strength but also impede toughness and mechanicalworkability. The amount of the carbide-forming element to be added is inthe range of from 0.2 to 4.0% by weight, preferably from 0.5 to 2.5% byweight.

The individual elements have proper amounts of their own. They aredesired to satisfy the following relevant ranges in order that thecarbide may be prevented from coarsening and may be finely dispersed andprecipitated in the base matrix. The range for Ti is from 0.1 to 1.0% byweight, that for Zr from 0.1 to 1.0% by weight, that for Hf from 0.1 to3.0% by weight, that for V from 0.4 to 1.2% by weight, that for Nb from0.1 to 2.0% by weight, that for Ta from 0.1 to 4.5% by weight, that forCr from 0.2 to 6.0% by weight, that for Mo from 0.1 to 2.5% by weight,and that for W from 0.1 to 4.5% by weight.

The carbide-forming element in this invention is desired to have atleast 75%, preferably not less than 80%, and more preferably practically100%, thereof to be present in the form of a precipitated phase. This isbecause the carbide-forming element contained in a solid solution has anadverse effect on the thermal expansion coefficient. For the purpose ofenabling the carbide-depositing element practically wholly to be presentin the precipitation phase and not remain as a solid solution in thebase matrix, it suffices to calculate the limits of the amount of theelement on the basis of the composition of the carbide of each elementand add the carbide-forming element in an amount falling within thefound limits. In the case of titanium, for example, the carbide to beformed is TiC. Since the density ρ_(T1) of titanium is 4.54 gr/cm³ andthe density ρ_(C) of carbon is 2.25 gr/cm³ and the density of Ti isabout 2.0 times that of C, the amount of titanium to be added is desiredto be about 2.0 times the amount of the carbon which remains aftergraphitization. The amount of the residual carbon mentioned above isgenerally in the range of from 0.5 to 0.7% by weight. If the amount oftitanium to be added exceeds about 1.4%, therefore, the excess will forma solid solution in the base matrix substrate and increase the thermalexpansion coefficient. For the other elements, it is desirable to findlimits of their respective amounts and set their proper amounts ofaddition in the same manner as described above. By thus setting theamounts of the carbide-forming elements to be added, the amounts ofrelevant solid solutions are extremely decreased and the property of lowexpansion is not affected.

For the method of this invention, the amount of the precipitated carbideis desired to be in the range of from 0.3 to 20% in terms of area ratioin the metal structure. If the area ratio of the precipitated carbide isless than 0.3%, the method will produce no sufficient effect onstrength, hardness, cuttability and workability, and property of lowexpansion. If it exceeds 20%, the thermal expansion coefficient andhardness of the carbide will bring about adverse effects and degrade theproperty of low expansion and cuttability and workability. The arearatio of the precipitated carbide is desirably in the range of from 0.5to 10%, and more desirably from 1.5 to 5.0%.

The grain size of the carbide also affects mechanical properties andcuttability and workability. The grain size of the carbide which isdesired to be in the range of from 5 to 50 μm can be controlled byadjusting the amount of carbon and the content of the carbide-formingelement. The aforementioned ranges of the amounts of the components ofthe low-expansion cast iron have been fixed with consideration to thefast just mentioned.

Then, the about of the nodular graphite precipitated in thelow-expansion cast iron of the present invention is desired to be in therange of from 0.5% to 15% in terms of area ratio in the metal structure.If the amount of the precipitated nodular graphite exceeds 15%, theexcess will exert an adverse effect on the strength of the cast iron.The upper limit of this range is desired to be 10%. The upper limit ofthe amount of carbon, therefore, is fixed at 2.5%.

In this invention, the area ratio mentioned above is determined by thefollowing method.

First, a photomicrograph of a ground cross section of a givenlow-expansion cast iron sample will be prepared. The cross section isetched with an aqueous 10% aqua regia solution to vivify the state ofprecipitation of the carbide. The photomicro-graph is desired to beobtained at 20 magnifications. The area ratio is defined by thefollowing formula:

    Area ratio, %, of the amount of precipitated carbide=Total area of carbide/(total area of base matrix+total area of carbide+total area of graphite)

The total areas of carbide and graphite have been recently determined byexamining a given photomicrograph by the use of an image analyzingdevice. A photograph magnified to a size of not less than 300 mm×200 mmis cut into areas of carbide, graphite, and base matrix. The areas ofphotograph are weighed and the area ratios are calculated on the basisof the weights thus found.

Now, the heat treatment will be described.

The heat treatment performed in this invention is primarily aimed atforming secondary graphite when the amount of carbon is relatively smalland the cast iron as cast permits either no or only insufficientcrystallization of graphite. With the composition of the cast iron ofthe formulation of this invention having a carbon content in the rangeof from 0.3 to 1.0%, the cast structure has the carbide onlyprecipitated and dispersed in the austenite base matrix or only a verysmall amount of graphite formed therein. Thus, the cast iron isdeficient in cuttability and workability. By subjecting this cast ironto a solid solution treatment at a temperature in the range of from 750to 900° C., the formation of the secondary graphite is attained. Thetime used for the solid solution treatment depends on the wall thicknessof the cast iron to be produced. The time which is calculated inaccordance with the following formula serves as the standard.

    Time for solid solution treatment=Largest wall thickness/25 mm×2 hours+2 hours

The range of the temperature of the solid solution treatment is set asmentioned above because the carbide is decomposed at temperaturesexceeding 900° C. and the amounts of dissolved carbon andcarbide-forming element are consequently increased and the thermalexpansion coefficient is increased rather than decreased.

In the method of this invention for the production of a high-strengthlow-expansion cast iron, a structure having the carbide dispersed andprecipitated is obtained even by the ordinary steps of melting andcasting. A structure having the carbide more uniformly and finelydispersed and precipitated can be obtained by a heat treatment. Sincethis treatment consists in rapidly cooling a melt from a hightemperature, it is employed only when the shape, wall thickness, etc. ofthe product have no problem as mentioned above. To be more specific,after the components for an alloy are melted and cast, the cast alloy issubjected to the solid solution heat treatment at a temperature in therange of from 750 to 900° C., and the hot alloy is rapidly cooled insuch a hardening medium as water, oil bath, or salt bath. As a result,there is obtained a structure in which the Ni segregation is cancelledand the carbide is finely dispersed. In a structure having the carbidefinely divided and dispersed therein as described above, the exaltationof strength is effectively attained. For example, the produced structuremanifests tensile strength of not less than 55 kgf/mm² and hardness(Brinnel hardness) of not less than HB 220 while maintaining a thermalexpansion coefficient of not more than 5×10⁻⁶ /° C. (at temperatures inthe range of from room temperature to 100° C.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical photomicrograph showing the metal structure of aconventional low-expansion alloy having graphite alone precipitatedtherein.

FIG. 2 is an optical photomicrograph showing the metal structure of alow-expansion alloy of this invention having a carbide alone dispersedtherein.

FIG. 3 is an optical photomicrograph showing the metal structure of alow-expansion alloy of this invention having a carbide and graphitedispersed therein.

FIG. 4A is an explanatory diagram showing one example of theconstruction of a silicon wafer polishing surface plate according tothis invention.

FIG. 4B is a perspective view showing one example of the polishingsurface plate of this invention shown in FIG. 4-A.

FIG. 5 is a schematic diagram showing the construction of a laseroscillator using a laser oscillator grade rod of this invention.

FIG. 6 is a schematic diagram for aiding in the explanation of the laseroscillator grade rod according to this invention.

FIG. 7 is a diagram showing the relation between the total carboncontent and mechanical properties of a conventional low-expansion castiron.

FIG. 8 is a diagram showing the relation between the total carboncontent and the dissolved carbon content as found in a low-expansioncast iron in one working example of this invention.

BEST MODE FOR EMBODYING THE INVENTION

Now, the present invention will be described below with reference toworking examples.

Examples 1 to 12 and Comparative Examples 1 to 5

Varying cast iron materials having a formulation shown in Table 2-1 weremelted by the use of a high-frequency electric furnace having a capacityfor 100 kg. The resultant melt was cast in a sand mold to produce a castiron sample measuring 25 mm×150 mm×200 mm and weighing about 6 kg. Thesamples of Examples 1 to 12 and Comparative Examples 1 to 5 were as castand not subjected to a heat treatment and were severally tested forthermal expansion coefficient (room temperature to 100° C.), tensilestrength, Young's modulus, Brinnel hardness HB, and amount ofprecipitated carbide in the metal structure. The test results are shownin Table 2-2.

The formulations of Examples 1 to 12 were the sums of fundamentalcompositions conforming to the present invention and suitable amounts ofcarbide-forming elements used either singly or in the form of a mixtureof two or more members. In contrast, in Comparative Examples 1 to 5, theformulations of Comparative Example 1 and 2 avoided containing acarbide-forming element, the formulation of Comparative Example 3contained a carbide-forming element in an excess amount, the formulationof Comparative Example 4 contained nickel and other elements in acomposition different from the basic composition of this invention, andthe formulation of Comparative Example 5 contained a carbide-formingelement in an unduly small ratio.

In these comparative examples, Comparative Example 5 was produced fromthe composition proposed in JP-A-62-205244 containing 0.02% of Nb and0.2% of V. The cast iron consequently produced showed virtually no signof formation of a carbide or no sign of improvement in strength.

It is clearly noted from the test results shown in Table 2-2 thatlow-expansion cast iron samples from formulations of the workingexamples of this invention showed thermal expansion coefficients of notmore than 7×10⁻⁶ /° C., amounts of precipitated carbide in metalstructure (area ratio) in the range of from 0.5 to 15%, tensile strengthof not less than 62 kgf/mm², and HB levels of hardness of not less than280 notwithstanding they were products which had been only cast and notsubjected to a heat treatment.

The cast iron samples of Comparative Examples 1, 2 and 5 which containedno or only a small amount of carbide-forming element showed smallamounts of precipitated carbide of not more than 0.2% and tensilestrength of not more than 45 kgf/mm². The cast iron sample ofComparative Example 3 which contained a carbide-forming element in anamount (7%) exceeding the upper limit of the range contemplated by thisinvention precipitated a carbide in a large amount and, owing to solidsolution of an unreactioned components, showed such a high thermalexpansion coefficient as 12×10⁻⁶ /° C. The cast iron sample ofComparative Example 4 which contained component elements in amountsdeviating from the ranges contemplated by this invention separated acarbide in a ratio exceeding 3% and showed a high thermal expansioncoefficient of 8.5×10⁻⁶ /° C.

When the metal metal structures of the cast iron samples of the workingexamples were examined under a microscope, it was confirmed that theyinvariably precipitated carbides uniformly and finely. As examples ofthese metal structure, an optical photomicrograph (200 magnifications)of the cast iron sample of Example 2 is shown in FIG. 2 and an opticalphotomicrograph (200 magnifications) of the cast iron sample ofComparative Example 1 is shown in FIG. 1. FIG. 1 shows only dispersionof nodular graphite and shows no sign of presence of carbide particles.FIG. 2 shows precipitation carbide particles of NbC and shows no sign ofpresence of graphite. The sample of Comparative Example 1 showed noprecipitation of a carbide and that of Example 2 showed precipitation ofa carbide and virtually no precipitation of graphite. The carbideparticles which were precipitated at all in these samples invariably hadsmall diameters of not more than 10 μm.

Examples 13 to 15 and Comparative Examples 6 to 10

Examples 13 to 15 represent the cases of giving a heat treatmentadditionally to the cast iron samples which were obtained exclusively bycasting respectively in Examples 1, 2, and 12. These examples underwenta procedure which comprised a heat treatment performed at a temperaturein the range of from 800 to 900° C. for about four hours, a solutionheat treatment, and a water-cooling treatment. In the resultant productsas cast, the excess dissolved carbon was transformed into secondarygraphite by the solid solution treatment and Ni and Co were uniformlydistributed by the rapid cooling. Particularly in the cast iron samplesof Examples 13 and 14 which had carbon contents of not more than 1.0%,though the samples of Examples 1 and 2 produced no sufficientcrystallization of graphite, the aforementioned procedure of heattreatments increased the amounts of graphite and, at the same time,slightly increased the amounts of precipitated carbides and decreasedthe amounts of dissolved carbon. Thus, Examples 13 to 15 lowered thermalexpansion coefficients and increased tensile strength as compared withthe properties exhibited by the samples of Examples 1 and 2. When thecarbon contents were not more than 0.8%, the secondary graphite assumeda spheroidal shape and the strength was amply high notwithstanding thecontent of Mg or Ca as a graphite spheroidizing element was not morethan 0.03%.

FIG. 3 is an optical photomicrograph (200 magnifications) showing a castiron sample of Example 14. This photograph shows the presence of bothNbC particles and nodular graphite in the metal structure. The carbideparticles having diameters of up to the maximum of 10 μm are observed tobe uniformly dispersed and deposited in the metal structure. The nodulargraphite had particle diameters ranging from 30 μm to 70 μm.

The sample of Example 15 had a high Co content of 11% as compared withthe samples of the other examples. It required a heat treatment foruniformizing cobalt. Owing to this heat treatment, the thermal expansioncoefficient was notably lowered as compared with the sample of the sameformulation obtained in Example 12 as cast.

Comparative Examples 6 to 8 used the same heat treatments as in Examples13 to 15 respectively. The temperature of solution heat treatment,however, was 850° C. in Comparative Example 6 and 950° C. in ComparativeExample 8. Comparative Example 6 represented a case of adding acarbide-forming element in an unduly small amount. In this case, nocarbide was formed notwithstanding a heat treatment was carried out.Comparative Example 7 represented a case of using a cobalt content ofnot less than 12% by weight. The sample obtained as cast failed toacquire a property of low expansion as desired. Comparative Example 8represented a case of performing a solution heat treatment attemperatures in the range of from 900 to 1000° C., a level enough forthoroughly decomposition of a carbide. With the effect of rapid coolingas a contributory factor, the sample obtained a satisfactory thermalexpansion coefficient of 2.8×10⁻⁶ /° C. Since this sample produced noprecipitation of a carbide, it was inevitably deficient in suchmechanical properties as tensile strength and hardness. ComparativeExamples 9 and 10 represented cases of producing samples as cast withoutusing a heat treatment. In these cases which used Si in amountsexceeding the upper limit of the range contemplated by this invention,the samples showed no sign of precipitation of a carbide-and weredeficient in mechanical strength.

Example 16

This example concerned a polishing surface plate using a high-strengthlow-expansion cast iron of this invention. FIG. 4-A is an explanatorydiagram showing schematically the construction of a polishing surfaceplate for use in the mechanochemical polishing of a silicon wafer as asemiconducting substrate. FIG. 4-B is a perspective view showing oneexample of the polishing surface plate. In the diagram, 1 stands for anupper surface plate, 2 for a lower surface plate, 3 for an abrasiveslurry feed pipe, and 4 for a wafer for polishing. In a high-frequencyelectric furnace having a capacity for 5000 kg, 4000 kg of cast iron ofa formulation shown in Table 3 was melted. A polishing surface plateshaped as shown in FIG. 4-B was produced by casting the melt of castiron with a sand mold. The resultant cast product was cut to obtain afinished surface plate 1000 mm in diameter and 40 mm in thickness.Generally, it is extremely difficult for a plate shaped like this toretain the flatness of its shape intact during the hardening treatment.For the sake of stable retention of the flatness of shape, this plate isrequired to be a product as cast. The cast product, as shown in Table 3,exhibited highly desirable properties such as thermal expansioncoefficient of 1.0×10⁻⁸ /° C., tensile strength of 70 kg/mm², andhardness of HB 300. The cast iron surface plate of the present examplewhich was obtained without a heat treatment showed Young's modulus about1.5 times that of the conventional brass surface plate and thermalexpansion coefficient about 1/20 times that of the brass surface plateand produced only small flexure under own weight. Further, with thiscast iron surface plate, the yield of silicon wafers having LTV valuesof the flatness of not more than 1.0 μm was about 1.5 times that of theconventional brass surface plate. The expression "LTV value of theflatness being 1.0 μm" used above means that the difference between thelargest and the smallest wall thickness within a given area of 15 mm×15mm taken on a polished wafer surface was not more than 1.0 μm.Separately, a surface plate of 550 mm in diameter was produced byrepeating the procedure of the present example described above. The castproduct of this size exhibited highly desirable properties as shown inTable 3 when it was given a heat treatment and further relieved ofresidual stress.

Example 17

This example concerned a laser oscillator grade rod according to thisinvention. FIG. 5 is a schematic diagram showing the construction of alaser oscillator using the rod of this invention. In the diagram, 1stands for an oscillation tube (quartz tube), 2 for an outlet mirror, 3for a rear mirror, 4 for a heat exchanger, and 5 for a rod. FIG. 6 is anexplanatory diagram showing a process for casting the rod mentionedabove. The laser oscillator grade rods are parts for determining thelength of a resonator which directly bears on the control of thefrequency of a laser. The relation between the frequency f of the laserand the length L of the resonator is expressed by the following formula:f=nC/2L.

In the formula, f stands for frequency, n for an integer, C for speed oflight, and L for length of the resonator. The variation ΔL of the lengthof the resonator, therefore, depends on the variation Δf of thefrequency of the laser in the relation of the following formula:Δf/f=ΔL/L

For the purpose of keeping the variable Δf of the frequency of the laserat a low level (below some hundreds of nm), it is necessary that thevariable ΔL of the length L of the resonator be repressed to a lowlevel. The rod of an oscillator is a part for fixing the length of theresonator. For the sake of permitting control of the change oftemperature, the rod is formed of a hollow pipe so constructed as to becooled with water. The laser oscillator grade rod constructed asdescribed above was produced by melting cast iron of the samecomposition as used in Example 16 in the same high-frequency electricfurnace and casting the melt by the use of a core 7 necessary for ahollow space and a mold 6 as shown in FIG. 6. The rod 5 of a lengthpractically equal to the length L of the resonator measured about 1000mm in length, 40 mm in outside diameter, and 20 mm in inside diameterand excelled in castability and in cuttability and workability as well.The hole in this rod 5 was formed by means of a cast borer using thecore 6 shown in FIG. 6 and finished by cutting. As a result, the rodobtained as cast attained thermal expansion coefficient of 1.0×10⁻⁶ /°C. at temperatures in the range of from room temperature to 100° C. It,therefore, could avoid forming deformation and residual stress due to aheat treatment. Since this rod possessed high rigidity, it could repressthe deflection below 0.1 mm. By the adoption of the laser oscillatorgrade rod of this invention constructed as described above, the ratio ofvariation Δf/f of the resonator could be stabilized to the order of1×10⁻⁶ because the variation of temperature could be controlled towithin 1° C.

Industrial Applicability

As described above, the method of this invention for the production of ahigh-strength low-expansion cast iron allows manufacture of cast ironpossessing improved strength, hardness, and cutting workability whileretaining the property of low expansion intact. This invention,therefore, permits provision of cast iron adapted for machine partswhich necessitate the property of low expansion and require the abilityto retain shape and resist abrasion. Further, the polishing surfaceplate contemplated by this invention is such that the polishing surfaceplate having a large size and using high-strength low-expansion castiron of this invention can be produced exclusively by casting withoutrequiring any heat treatment. As respects the laser oscillator graderod, the rod produced by using high-strength low-expansion cast iron ofthis invention is allowed to attain low thermal expansion coefficientand high rigidity without requiring any heat treatment and repress theratio of variation of the frequency of a resonator to a low level.

In addition to the silicon wafer polishing surface plate and the laseroscillator grade rod mentioned above, the high-strength low-expansioncast iron of this invention can be adapted for various applicationsmaking use of property of low expansion, strength, hardness, and cuttingworkability such as, for example, laser grade spherical polishingsurface plate, metal die for CFRP parabolic antenna, stand for laseroscillator, stand for long-distance transmission of laser, laserreflecting plate, optical part holder, solder printer roller,microgauge, and other similar precision mechanical parts.

                  TABLE 1-1                                                       ______________________________________                                                        Alloy Composition (weight %)                                                  C      Si   Mn   Ni    Co    Fe                               ______________________________________                                        1.  Inver       --     --   --   34-36 --    Balance                          2.  Sper Inver  --     --   --   30-33 4-6   Balance                          3.  Niresist D5 ≦2.4                                                                          1.0- ≦1.0                                                                        34-36 --    Balance                                                 2.8                                                    4.  Nobinite    0.8-   1.0- 0.4- 30-33 4-6   Balance                              Cast Iron   3.0    3.0  2.0                                                   (JP-A-60-51547)                                                           5.  Cast Iron   1.0-   ≦1.5                                                                        ≦1.5                                                                        32-   1.0-4.0                                                                             Balance                              (JP-A-62-268249)                                                                          3.5              39.5                                         ______________________________________                                    

                  TABLE 1-2                                                       ______________________________________                                                         Thermal                                                                       Extension                                                                     Coefficience Tensile                                                          (0-100° C.) ×                                                                 Strength                                                                             Hardness                                                  10.sup.-6 /° C.                                                                     kgf/mm.sup.2                                                                         HB                                       ______________________________________                                        1.     Inver     1.5          40-45  120                                      2.     Super     0.5          40-45  120                                             Inver                                                                  3.     Niresist D5                                                                             5            40-45  120                                      4.     Nobimite  4            40-45  120                                             Cast Iron                                                              5.     JP-A-62-  2            45-55  120                                             268249                                                                 ______________________________________                                    

                  TABLE 2-1                                                       ______________________________________                                        Component *1 (weight %)                                                                                              Carbide-forming                             C      Si    Mn  Ni   Co   Mg + ca                                                                              element                                ______________________________________                                        Example                                                                       1    0.3    0.2   0.1 29.0 4.6  0      Ti 0.6                                 2    0.8    0.2   0.1 29.5 4.5  0.03   Nb 1.0                                 3    1.2    0.4   0.1 28.7 5.4  0.05   Ta 2.0                                 4    1.0    0.8   0.1 36.0 0    0.07   Nb 1.0, Zr 0.5                         5    1.4    0.3   0   33.5 2.5  0.05   V 2.3                                  6    1.5    0.3   0   28.5 5.6  0.05   Nb 1.5, Hf 0.1                         7    0.9    0.5   0.2 38.5 0    0.04   W 0.3, Mo 0.3                          8    1.5    0.3   0.2 29.5 4.0  0.04   Cr 3.5                                 9    2.0    0.8   0.5 30.3 4.8  0.05   Nb 5.3                                 10   2.0    0.8   0.4 29.6 5.2  0.05   Ti 0.2, Nb 0.2, Ta                                                            0.2, Zr 0.4, V 0.5, Hf                                                        0.2,                                   11   2.4    1.1   0.9 29.6 4.3  0.04   Nb 1.4                                 12   1.2    0.2   0.2 25.0 11.0 0.04   Ti 0.5, Nb 0.5                         13   0.3    0.2   0.1 29.0 4.6  0      Ti 0.6,                                14   0.8    0.2   0.1 29.5 4.5  0.03   Nb 1.0                                 15   1.2    0.2   0.2 25.0 11.0 0.04   Ti 0.5, Nb 0.5                         Comparative Example                                                           1    0.8    0.2   0.1 29.5 5.0  0.04   --                                     2    2.0    0.8   1.5 31.0 4.5  0.04   --                                     3    2.5    0.3   0.5 30.1 5.0  0.04   Ti 1.0, V 3.0, W 2.0                                                          Mo 1.0                                 4    1.5    0.4   1.2 23.0 4.8  0.05   Ti 1.0, Nb 1.0                         5    3.0    1.5   1.0 30.0 3.0  --     Nb 0.02, V 0.2                         6    3.0    1.0   1.0 30.0 3.0  0.05   Ti 0.02, V 0.2                         7    0.8    1.2   0.2 28.5 14.0 0.05   Nb 0.4                                 8    0.8    0.7   0.4 32.0 4.9  0.04   Nb 0.7, Cr 0.7                         9    2.25   2.2   0.1 31.9 4.6  0.02   Ti 0.1, Cr 0.03                        10   2.05   1.9   0.3 36.5 0    0.02   Ti 0.13, Cr 0.3                        ______________________________________                                         *1: The rest is Fe, which includes inevitable impurities.                

                  TABLE 2-2                                                       ______________________________________                                                                               Area                                               Thermal                    Ratio of                                                                            Dis-                                         Expansion                  Carbide                                                                             solved                               Heat    Co-      Tensile                                                                             Young's                                                                              Hard-                                                                              Precipi-                                                                            Corbon                               Treat-  efficient ×                                                                      Strength                                                                            Modulus                                                                              ness tation                                                                              Content                              ment    10.sup.-6 /° C.                                                                 kg/mm.sup.2                                                                         kg/mm.sup.2                                                                          HB   %     Wt. %                            ______________________________________                                        Example                                                                       1   No      1.7      57    16000  280  0.5   0.22                             2   No      2.3      72    17000  320  2.0   0.40                             3   No      0.9      70    16200  300  3.0   0.15                             4   No      1.4      72    16800  300  2.0   0.20                             5   No      2.3      65    16500  296  2.0   0.38                             6   No      2.0      62    15400  288  2.5   0.36                             7   No      1.9      80    16000  330  1.0   0.25                             8   No      1.5      75    16000  360  3.5   0.21                             9   No      6.2      70    16000  360  4.0   0.40                             10  No      5.9      64    16400  420  15.0  0.39                             11  No      5.7      67    16500  300  2.0   0.36                             12  No      7.0      65    15800  300  1.6   0.40                             13  Yes     0.8      64    15900  280  0.7   0.19                             14  Yes     0.9      75    16200  300  2.2   0.16                             15  Yes     0.6      69    16000  300  3.4   0.14                             Comparative Example                                                           1   No      1.2      45    16000  170  0     0.47                             2   No      7.4      42    16000  186  0.2   0.52                             3   No      13.0     62    17000  500  21    0.15                             4   No      8.5      65    15300  300  3.6   0.25                             5   No      2.0      35    14000  130  0     0.42                             6   Yes     7.0      32    12000  200  0     0.35                             7   No      12.0     65    14000  320  0     0.38                             8   Yes     2.8      50    13600  240  0     0.45                             9   No      5.0      32    11500  132  0     0.49                             10  No      4.4      35    12300  125  0     0.52                             ______________________________________                                         Note 1:                                                                       Example 13, 14, 15, and Comparative Example 6 and 7 used the heat             treatment material. (The temperature of solution heat treatment is            850° C.)                                                               Note 2:                                                                       Comparative Example 8 used the heat treatment material. (The temperature      of solution heat treatment is 950° C.)                            

                  TABLE 3                                                         ______________________________________                                        Example 16                                                                    Component (weight %)                                                          C    Si    Mn      Ni   Co    Mg   Ti    Nb  Fe                               ______________________________________                                        1.2  0.2   0.1     29.5 4.6   0.05 0.3   0.4 Balance                          ______________________________________                                        Properties of cast Products                                                         Thermal                          Area Ratio                             Heat  Extension Tensile  Young's       of Carbide                             Treat-                                                                              Coefficient ×                                                                     Strength Modulus                                                                              Hardness                                                                             Precipitation                          ment  10.sup.-6 /° C.                                                                  kgf/mm.sup.2                                                                           kgf/mm.sup.2                                                                         HB     %                                      ______________________________________                                        No    1.0       70       16000  300    2.5                                    Yes   0.7       66       15800  280    2.8                                    ______________________________________                                         Note:                                                                         Fe includes inevitable impurities.                                       

What is claimed is:
 1. A method for the production of low-expansion castiron of a high Ni content exhibiting a coefficient of thermal expansionof not more than 8×10⁻⁶ /° C. at temperatures in the range of from roomtemperature to 100° C., comprising the steps of preparing a materialconsisting of not less than 0.3% by weight to not more than 2.5% byweight of C, not more than 0.1% by weight of Mg or Ca, not less than 25%by weight to not more than 40% by weight of Ni, less than 12% by weightof Co, not less than 0.1% by weight to not more than 6.0% by weight of acarbide-forming element, and the balance of Fe and other inevitableimpurities, melting said material, and casting the melt in a mold of astated shape, and enabling said carbide-forming element, while said meltis being solidified in said mold, to be finely dispersed andprecipitated in a base matrix in the form of carbide particles at anarea ratio in the range of from 0.3% to 20% in the metal structure,simultaneously with graphite.
 2. The method according to claim 1,wherein said material for cast iron further comprises not more than 1.2%by weight of Si and not more than 1.0% by weight of Mn.
 3. The methodaccording to claim 1, wherein said carbide-forming element is at leastone member selected from the group consisting of the transition metalelements of IVa, Va, and VIa Groups in the Periodic Table of theElements.
 4. The method according to claim 1, wherein the step of heattreatment after the step of casting is not included.
 5. A method for theproduction of low-expansion cast iron of a high Ni content exhibiting acoefficient of thermal expansion of not more than 8×10⁻⁶ /° C. attemperatures in the range of from room temperature to 100° C.,comprising the steps of preparing a material consisting of not less than0.3% by weight to not more than 2.5% by weight of C, not more than 0.1%by weight of Mg or Ca, not less than 25% by weight to not more than 40%by weight of Ni, less than 12% by weight of Co, not less than 0.1% byweight to not more than 6.0% by weight of a carbide-forming element, andthe balance of Fe and other inevitable impurities, melting saidmaterial, and casting the melt in a mold of a stated shape, and enablingsaid carbide-forming element, while said melt is being solidified in themold, to be finely dispersed and precipitated in a base matrix in theform of carbide particles in the metal structure thereby lowering thecontent of dissolved carbon in said cast iron to not more than 0.4% byweight.
 6. The method according to claim 5, wherein said material forcast iron further comprises not more than 1.2% by weight of Si and notmore than 1.0% by weight of Mn.
 7. The method according to claim 5,wherein said carbide-forming element is at least one member selectedfrom the group consisting of the transition metal elements of IVa, Va,and VIa Groups in the Periodic Table of the Elements.
 8. The methodaccording to claim 5, wherein graphite is simultaneously dispersed withsaid carbide in said metal structure.
 9. The method according to claim5, wherein not less than 75% of the amount of said carbide-formingelement incorporated is precipitated in the form of a carbide in saidmetal structure of cast iron.
 10. The method according to claim 5,wherein the step of heat treatment after the step of casting is notincluded.
 11. A method for the production of a high-strengthlow-expansion cast iron, comprising the steps of preparing a materialfor cast iron consisting of not less than 0.3% by weight to not morethan 2.5% by weight of C, not more than 0.1% by weight of Mg or Ca, notless than 25% by weight to not more than 40% by weight of Ni, less than12% by weight of Co, not less than 0.1% by weight to not more than 6.0%by weight of a carbide-forming element, and He balance of Fe and otherinevitable impurities, melting said material, enabling saidcarbide-forming element, while said melt is being cast and solidified,to be finely dispersed and precipitated in a base matrix as carbideparticles in the metal structure of cast iron and, at the same time,lowering the dissolved carbon content contained in the cast iron andgiving rise to cast iron exhibiting a coefficient of thermal expansionof not more than 8×10⁻⁶ /° C. at temperatures in the range of from roomtemperature to 100° C. and tensile strength of not less than 55 kgf/mm².12. The method according to claim 11, wherein said cast iron hashardness of not less than HB
 200. 13. The method according to claim 11,wherein said material for cast iron further comprises not more than 1.2%by weight of Si and not more than 1.0% by weight of Mn.
 14. The methodaccording to claim 11, wherein said carbide-forming element is at leastone member selected from the group consisting of the transition metalelements of IVa, Va, and VIa Groups in the Periodic Table of theElements.
 15. The method according to claim 11, wherein the content ofsaid carbide is in the range of from 0.3% to 20% in terms of area ratioin said metal structure.
 16. The method according to claim 11, whereinthe amount of dissolved carbon contained in said cast iron is not morethan 0.4% by weight.
 17. The method according to claim 11, wherein thestep of heat treatment after the step of casting is not included.
 18. Amethod for the production of a polishing surface plate of high-strengthlow-expansion cast iron, comprising the steps of preparing a materialfor cast iron consisting of not less than 0.3% by weight to not morethan 2.5% by weight of C, not more than 0.1% by weight of Mg or Ca, notless than 25% by weight to not more than 40% by weight of Ni, less than12% by weight of Co, not less than 0.1% by weight to not more than 6.0%by weight of a carbide-forming element, and the balance of Fe and otherinevitable impurities, melting said material, enabling saidcarbide-forming element, while said melt is being cast and solidified,to be finely dispersed and precipitated in a base matrix as carbideparticles in an area ratio in the range of from 0.3% to 20% in the metalstructure of cast iron.
 19. The method according to claim 18, whereinsaid material for cast iron further comprises not more than 1.2% byweight of Si and not more than 1.0% by weight of Mn.
 20. The methodaccording to claim 18, wherein said carbide-forming element is at leastone member selected from the group consisting of the transition metalelements of IVa, Va, and VIa Groups in the Periodic Table of theElements.
 21. The method according to claim 18, wherein said polishingsurface plate has a diameter of not less than 600 mm.
 22. The methodaccording to claim 18, wherein the step of heat treatment after the stepof casting is not included.
 23. The method according to claim 18,wherein the amount of dissolved carbon contained in said metal structureof cast iron is not more than 0.4% by weight.
 24. The method accordingto claim 18, wherein said cast iron exhibits a coefficient of thermalexpansion of not more than 8×10⁻⁶ /° C. and tensile strength of not lessthan 55 kgf/mm².
 25. A method for the production of a laser oscillatorgrade rod, comprising the steps of preparing a material for cast ironconsisting of not less than 0.3% by weight to not more than 2.5% byweight of C, not more than 0.1% by weight of Mg or Ca, not less than 25%by weight to not more than 40% by weight of Ni, less than 12% by weightof Co, not less than 0.l by weight to not more than 6.0% by weight of acarbide-forming element, and the balance of Fe and other inevitableimpurities, melting said material, enabling said carbide-formingelement, while said melt is being cast and solidified, to be finelydispersed and precipitated in a base matrix as carbide particles in anarea ratio in the range of from 0.3% to 20% in the metal structure ofcast iron.
 26. The method according to claim 25, wherein said materialfor cast iron further comprises not more than 1.2% by weight of Si andnot more than 1.0% by weight of Mn.
 27. The method according to claim25, wherein said carbide-forming element is at least one member selectedfrom the group consisting of the transition metal elements of IVa, Va,and VIa Groups in the Periodic Table of the Elements.
 28. The methodaccording to claim 25, wherein the step of heat treatment after the stepof casting is not included.
 29. The method according to claim 25,wherein the amount of dissolved carbon contained in said metal structureof cast iron is not more than 0.4% by weight.
 30. The method accordingto claim 25, wherein said cast iron exhibits a coefficient of thermalexpansion of not more than 8×10⁻⁶ /° C. and tensile strength of not lessthan 55 kgf/mm².
 31. The method according to claim 1, wherein the grainSize of said carbide particles in the metal structure is not more than10 μm.
 32. The method according to claim 5, wherein the grain size ofsaid carbide particles in the metal structure is not more than 10 μm.33. The method according to claim 11, wherein the grain size of saidcarbide particles in the metal structure is not more than 10 μm.
 34. Themethod according to claim 20, wherein the grain size of said carbideparticles in the metal structure is not more than 10 μm.
 35. The methodaccording to claim 25, wherein the grain size of said carbide particlesin the metal structure is not more than 10 μm.